I'm going to select the "Remove only half of noise" preset.



Then I'm going to click the "Apply" button near the top to apply the filter.



The noise in the sky is gone, but the rest of the image looks kind of "plasticy" with a loss of detail. I don't want that. This tends to happen when you overdo it with a noise reduction filter.



The solution to this problem is to erase the parts of the image in the top layer that I don't want affected by the noise reduction filter. I'll just select the "Eraser" tool in the toolbar, sixth tool down on the left (or just hit the "E" key), and I'll go ahead and erase where I want to have the detail back.